This invention relates generally to electrically mediated pain relief and, in particular, to the use of electrical current to block sensory pathways. Particular emphasis is given to the medial branch of the spinal nerve root and its articular branches so as to relieve pain caused by painful zygopophysial joints, but the invention can be applied to any portion of the nervous system, including central, autonomic, or peripheral.
Pain impulses are transmitted through the peripheral nervous system. Although electrical stimulators have been used to control pain through depolarization of the nerve, this type of treatment is limited by the uncomfortable and often painful effects of the depolarizing electrical stimuli. Indeed, pain from such treatment limits the amount of electrical current that can be applied to the nerve and consequently also limits the amount of pain relief that can be achieved.
The vascular system is modulated by the autonomic nervous system. It has been shown that stimulation of this system at the level of the spinal cord with depolarizing current can improve anginal cardiac pain, vasospasm and arterial insufficiency. However, this type of depolarizing treatment (spinal cord stimulation) is also limited by the uncomfortable nature of the depolarizing current when applied in close proximity to the spinal cord.
A large body of evidence now exists to support the fact that the zygopophysial joints (facet joints) can be pain-producing structures. In particular, it has been shown that the facet joints can be a source of chronic spinal pain in the cervical, thoracic and lumbosacral regions. This pain, which can be due to trauma to and/or degeneration of the facet joint, can be disabling in some patients.
Anatomical dissections reveal that the facet joint is innervated by the articular branches of the medial branch of the spinal nerve. Lesioning this nerve has been shown to relieve pain, but regrowth of the nerve is inevitable and pain returns.
Electrical stimulators of other neural structures has been proposed to provide pain relief. U.S. Pat. No. 5,041,974 to Walker et al, entitled MULTICHANNEL STIMULATOR FOR TUNED STIMULATION, includes a user interface enabling the selection of a channel and the creation of a stimulus wave signal. A slave circuit associated with the channel receives the wave building signal and generates a corresponding low-power stimulus. An output circuit coupled to the slave circuit electronically isolates the stimulus from the other channels, amplifying and converting it to produce a high-fidelity stimulus wave signal.
There have also been developed neuro-type stimulators which are responsive to patient conditions, and adjust their function in accordance therewith. For example, in U.S. Pat. No. 5,031,618 to Mullett, entitled POSITION-RESPONSIVE NEURO STIMULATOR, a position sensor such as a mercury switch which may be used to determine whether a patient is erect or supine, is implanted in the patient. This position information is then used to vary stimulation intensity, in terms of pulse amplitude, pulse width, and a number of pulses per second and other factors. The output of the pulse generator is applied to the spinal cord, peripheral nerves and/or targets in the brain with leads in electrodes in a manner consistent with a given medical need.
To Applicant""s knowledge, however, no such neuro stimulators, whether adaptive or fixed in their operation, have been used to hyperpolarize any part of the central, autonomic, or peripheral nervous system. Nor have such techniques been applied to the specific problem of pain relief relative to the zygopophysial or facet joints through depolarization or hyperpolarization of the medial branch nerves. Neuro- stimulators have yet to be used for anodal blockade of the nervous system or, more specifically, stimulation of the medial branches innervating the zygopophysial or facet joints. As the ""618 patent points out, stimulation of this type has so far been limited to the treatment of chronic intractable pain requiring spinal cord depolarization.
Accordingly, one object of this invention is to use anodal blockade of electrical nerve impulses in the central, autonomical, and peripheral nervous system.
A further object is to block painful impulses transmitted by the medial branch of the spinal nerve.
Another important object of this invention is to provide apparatus in the form of an electrical nerve stimulator that is implantable, as well as the accompanying electrodes, and methods of using the same.
It is a goal of this invention to create a pain-relief mechanism and accompanying methodology that is long lasting while being minimally invasive so as to reduce medical complications and provide improved pain relief as compared to temporary modalities.
One aspect of the present invention resides in a method of creating an anodal blockade of the central, autonomic, or peripheral nervous system, with particular emphasis being directed to hyperpolarizing a peripheral nerve or branch of the autonomic nervous system that is external to the spinal column. Although any peripheral nerve or portion of the autonomic nervous system that is external to the spinal canal may be the target of the anodal blockade, the sciatic nerve is of particular interest due to its location and relationship to the vascular system.
According to a preferred method of the invention, a positive electrode (anode) is placed in close proximity, preferably a few millimeters, relative to a target nerve. A negative electrode is placed in an area of low sensitivity at a point remote from the positive electrode, preferably in the adipose tissue. Both of these electrodes could lie under the skin, and may require surgical placement.
The electrodes are connected to a stimulator that generates an electrical current operative to induce the hyperpolarization of the nerve. The stimulator may be implanted under the skin, or may be located outside the body. The stimulator may further include a controller and appropriate electronics operative to generate electrical impulses tailored to an individual""s need for appropriate pain relief or modulation of the vascular system in terms of pulse frequency, pulse width, and pulse amplitude.
Another aspect of the present invention resides in methods and apparatus for stimulating the medial branch of the spinal nerve associated with a painful spinal facet joint, so as to block pain impulses from reaching the spinal cord whether through depolarization or hyperpolarization. Broadly, according to an apparatus aspect, the invention is comprised of a neurostimulator and two or more electrodes placed adjacent to the target nerve or nerves. In the preferred embodiment, the apparatus is capable of generating electrical impulses of sufficient intensity to cause stimulation of a given medial branch and its articular branches. Multiple leads with the same polarity may be placed near multiple pain-generating medial and articular branch nerves, with one or more electrodes of the opposite polarity being placed away from the target site. If the electrode adjacent the nerve is a negative electrode, the stimulation is a negative electrical pulse that depolarizes the nerve. If the electrode adjacent the nerve is a positive electrode, the stimulation is a positive electrical pulse which hyperpolarizes the nerve. In any case, the target nerve does not reach threshold and therefore never conducts an impulse.
Although the apparatus may be disposed externally of the individual, in the preferred embodiment the stimulator is physically small and battery operated, facilitating implantation underneath the skin. Accordingly, the components of the stimulator and electrodes are preferably biocompatible and biostable so as not to cause tissue reactions.
The stimulator includes a controller and appropriate electronics operative to generate electrical impulses tailored to an individual""s need for appropriate pain relief in terms of pulse frequency, pulse width, and pulse amplitude. In an alternative embodiment, the stimulator further includes electrodes and electrical circuitry operative to monitor myoelectrical activity generated by the surrounding muscles and modulate the impulses generated by the stimulator to meet the demands of the individual""s activity and/or to prolong battery life.